Coating Materials Containing Gold Nano-Particles

Gold 2003

Taizo Nanke, Toshikatsu Kobayashi,

Abstract
1. IntroductionNano-sized metal particles are of interest in a variety of fields including electronics1), optics2), and chemical catalysis3).We have developed a novel method to prepare concentrated and stable dispersions of gold nano-particles with diameters of 5 - 45 nm4). The method consists of two key technologies. The first is the protection of particles from their mutual coagulation. To do this, we have utilized a special kind of comb-shaped block copolymer to stabilize the particles. The second is the reduction method of the gold ions. We found that some amines reduce gold ions and yield nano-particles under industrially controllable conditions. Combining these two key technologies enabled us to prepare stable and highly concentrated pastes of gold nano-particles. Thus far, we have succeeded in preparing pastes containing up to 95 % metal particles. By selecting the polarity of the protective polymer, these pastes can be stably diluted or mixed with aqueous and non-aqueous media. In this paper, we will report on the utilization of the above concentrated gold nano-particle pastes.The first application was as a colorant. When the particle of gold's size reaches about 10 nm, their suspension exhibits a red color. The mechanism of coloring is attributed to plasmon absorption of free electrons within the metal particles2). The gold nano-particle has been known as an aesthetic red colorant for stained glass and fine glass-ware, such as Venetian glass. We intended to reproduce this elegant and stable coloring in paint films. The second application was metal-like film production. As the metal concentration within the pastes was very high, we expected that a simple coating and baking procedure could yield a thin metal film. 2. Experimental2.1 Application as colorantsA gold paste containing 40 % of the c.a. 10 nm nano-particles was used for the colorant application. A polyester based, comb-shaped block copolymer was also contained as the protective polymer. A TEM photograph of the gold nano-particles is shown in Fig. 1.The paste was mixed with an acrylic-melamine resin system to prepare a thermosetting nano-particle paint. The particle concentration within the solid components was 0.5 - 4 % by weight. The nano-particle paint was used as color clear coating. First, base coating containing aluminum flakes was applied on primed steel substrate, then the nano-particle paint was applied on it. After baking at 140 C for 20 minutes, the appearance of the coatings was visually inspected.2.2 Application to metal film formationFor metal film formation, an acrylic copolymer was used as the protective polymer. As the polarity of the polymer was higher than the polyester polymer used in the colorant application, the pastes were soluble in polar solvents such as water and C1 - C3 alcohols. Particle concentrations in the gold pastes were 85 - 95%. TEM photographs revealed that particle sizes were about 20 - 35 nm. The pastes were diluted by ethyl alcohol to adjust the viscosity and then coated on substrates. Air-spraying, spin-coating, and dip-coating were used as coating methods. Glass plate, PET film and china figures were used as substrates. After drying at room temperature or after baking at 250 C for 1 hour, the appearance of the coated films were visually inspected.3. Results and Discussion3.1. Application as colorantsThe painted panel had an aesthetic appearance which ordinary pigments could not give (Fig.2). Moreover, this paint had higher transparency and color-flop effect, which are specific properties of coatings colored by the gold nano-particles. 3.2 Application to metal film formulationDried film of the more highly concentrated gold paste exhibited a metal-like appearance without baking. This could be due to the fact that the metal particle concentration within the film was close to CPVC5). In the case of the lower concentrated gold paste, gold appearance appeared (Fig.3) after baking. In this case, the protective polymer seemed decomposed and eliminated from the films. Furthermore, these films showed electro-conductivity.References1) M. Oda, Hyomen Gijutsu, 47, 910 (1996)2) S. Ogawa et al., Jpn. J. Appl. Phys., 33, Pt. 2, No. 3A, L331 (1994)3) M. Haruta, Gendai Kagaku, No. 5, 42 (1998)4) T. Kobayashi, H. Kamo, Chem. & Chem. Ind., 53, 909 (2000)5) CPVC = Critical Pigment Volume Concentration